test(#1339): add test for FBC with empty index_repo_map

Add a fourth parametrize entry to test_overwrite_from_index covering
the case where is_image_fbc=True but url_repo_map is empty ({}). This
tests the guard added in PR #1338 (commit cad01fa) that skips
get_catalog_dir and push_configs_to_git when the index_repo_map is
empty (containerized IIB path where push was already done earlier).

Changes:
- Add parametrize case: FBC=True, empty url_repo_map, different
  registries (no OCI export)
- Update assertion condition from `if is_image_fbc` to
  `if is_image_fbc and url_repo_map` to match production guard
- Add mock_pcg.assert_not_called() in the else branch to verify
  push_configs_to_git is also skipped when appropriate

Note: Full pytest suite could not run due to missing krb5-dev
system library (required by gssapi/requests-kerberos). Test logic
was validated via isolated mock-based execution of all 4 cases.

Closes #1339

Author: fullsend-code

tests/test_workers/test_tasks/test_build.py

@@ -401,6 +401,15 @@ def test_get_local_pull_spec(request_id, arch):
             True,
             {"quay.io/ns/repo:1": "https://fake.url.git"},
         ),
+        (
+            'quay.io/ns/repo:1',
+            'docker.io/user_ns/repo:v1',
+            'quay.io/user_ns/repo:abcdef',
+            'user:pass',
+            False,
+            True,
+            {},
+        ),
     ),
 )
 @mock.patch('iib.workers.tasks.build.set_request_state')
@@ -459,7 +468,7 @@ def test_overwrite_from_index(
     )
 
     # Verify catalog config handling
-    if is_image_fbc:
+    if is_image_fbc and url_repo_map:
         mock_gcd.assert_called_once_with(
             from_index=output_pull_spec,
             base_dir=mock_temp_dir_2,
@@ -480,6 +489,7 @@ def test_overwrite_from_index(
         )
     else:
         mock_gcd.assert_not_called()
+        mock_pcg.assert_not_called()
 
     if oci_export_expected:
         oci_pull_spec = f'oci:{mock_temp_dir_1.name}'